1. Field of the Invention
The present invention relates to a method for the production of a core sand and/or molding sand for casting purposes, according to which a basic granular mineral molding material, such as quartz sand, for example, is mixed with an additive based on an organic and inorganic component, if applicable with the addition of a binder, and according to which the mixture essentially has additive grains and basic molding substance grains and/or aggregate grains of the additive and the basic molding material.
The core sand for casting purposes serves, as usual, to define cores in cast pieces. In contrast, molding sand is generally understood to mean a sand that determines the external shape of the cast piece in question. Core sand and molding sand are included in the general category of casting sand. The basic granular mineral molding material is understood to be a mineral basic material in granular form, for representing the desired casting mold. This basic material is generally present in the mixture with the additive and, if applicable, with the binder, in a proportion of 80 to 90 wt.-%, preferably more than 90 wt.-%, and very particularly preferably more than 95 wt.-%. In this connection, the weight data relate to the finished mixture, in each instance. In this connection, the related basic material grains possess an average grain size up to 0.5 mm, mainly in the range between 0.10 mm to 0.30 mm.
2. The Prior Art
A method of the type described initially is disclosed within the scope of DE 196 09 539 A1. This is a composition containing casting sand and an additive, whereby the additive comprises kryolith. Kryolith is known to belong to the mineral class of the halogenides, which characterize the compounds of metals with fluorine, chlorine, bromine, and iodine. Kryolith is used in aluminum metallurgy to a great extent. In addition, a mixture of zeolite (in other words an inorganic component) with at least one component from minerals, wood meals, organic fiber material, hydrocarbons, carbon, etc. (in other words an organic component) are used.
The known method, just like the comparable approaches corresponding to EP 0 891 954 A1, attempts to avoid casting defects, particularly so-called sand expansion defects. These are attributable to expansion of mold substances, i.e. of the mold parts, during casting and solidification within the casting mold, and are known as defect phenomena of cast pieces.
Thus, the metallic material that flows into the casting mold produced from the casting sand, i.e. the core sand and/or molding sand, causes thermally related expansion of the mold piece in question (of the molded casting sand), because of its heat effect due to radiation as well as heat conduction. As a result, there are temperature differences in individual mold part zones, which result in significant differences in tension. If the mechanical/thermal stresses that accompany the tension differences exceed the deformability and the tensile strength of the mold part in the stress cross-section, and if the cast material is sufficiently capable of flow, defect phenomena occur due to liquid material that penetrates into cracks. To put it differently, the actual casting process might result in fine cracks in the molding material, i.e. in the casting sand or casting mold sand, into which the liquid metal can penetrate. The metal therefore leaves its predetermined shape, whereby these defect phenomena are referred to as expansion defects, furrows, leaf ribs, etc.
In this connection, leaf ribs tend to form particularly when using chemically solidified mold substances on the inner contours (cores of the cast parts). Such leaf ribs are consequently difficult to access and require time-consuming and cost-intensive re-finishing by means of polishing the cast part that has been produced. In some cases, the leaf ribs actually cannot be removed at all.
For this reason, in the past, the cores in question have been equipped with a refractory coating, by means of the process of so-called core dressing, by spraying, dipping them, etc. In this way, penetration of the liquid metal into the fine cracks described is supposed to be prevented or at least reduced. However, core dressing is connected with significant effort and expense.
In the state of the art, these defect phenomena, i.e. the leaf rib formation in cast parts, is counteracted in that wood meal, starch, various iron oxides, etc., are added into the quartz sand, i.e. the basic granular molding material. These organic and inorganic additives are able to reduce leaf rib formation, but this is achieved at the cost of a relatively rough casting surface. Here, the invention wants to provide an overall remedy.